Method of producing fluorapatite

ABSTRACT

A method of producing fluorapatite wherein predetermined amounts of calcium monohydrogen phosphate, calcium carbonate, and calcium fluoride are weighed, subjected to a mechanochemical action, and caused to react with one another in water. With this method, a fluorapatite having a uniform composition can be obtained without the accompaniment of by-products. A moisture sensitive resistor of the present invention can be obtained by sintering the fluorapatite produced in accordance with this method. This moisture sensitive resistor has a sufficiently low resistance corresponding to a humidity, and has substantially no hysteresis.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of producing fluorapatite anda moisture sensitive resistor made of sintered body of the fluorapatiteobtained by the same.

2. Description of the Prior Art

Fluorapatite [Ca₁₀ (PO₄)₆ F₂ ; to be described as FAp hereinafter] is amain mineral constituting rock phosphate and used as a phosphormaterial.

FAp synthesizing methods are roughly classified into a dry method and awet method.

The dry method includes a step of sintering the raw materials (calciumphosphate and calcium fluoride; calcium pyrophosphate and calciumfluoride; and the like) of FAp at a temperature as high as severalhundreds of degrees of centigrade. Therefore, fluorine tends toevaporate easily, and it is difficult to obtain an FAp powder having auniform composition.

As the wet method, a method involving adding calcium fluoride to asuspension of calcium monohydrogen phosphate and causing the twomaterials to react to each other, a method hydrolyzing calciummonofluorophosphate dihydrate, and the like are known. However, thesemethods often accompany by-products. Therefore, it is difficult toobtain pure FAp with these methods.

It is also known as another method of obtaining FAp that, in the processof producing hydroxyapatite, fluorine ions mixed with materials ofhydroxyapatite. However, FAp obtained with this method is amorphous.

As still another method of obtaining FAp, a method of mixing calciumfluoride and α-calcium phosphate at a molar ratio of 1:3 and grindingthe two materials is known (Kogyo Kagaku Zasshi, Vol. 71, No. 9, (1968)p. 1307). However, with this method, the starting materials remainunreacted even after grinding for 8 hours, and it is difficult to causethe starting materials to completely react to each other.

The present invention also relates to a moisture sensitive resistor. Amoisture sensitive resistor exhibits a resistance corresponding to therelative humidity of the atmosphere.

Moisture sensitive resistors using ceramics or polymer films are known.However, the electric resistance of a moisture sensitive resistor ofthis type corresponding to the humidity is high. Particularly, under alow humidity conduction, the electric resistance exceeds the upper limitof the practical measurement range (10 MΩ or less) or the sensitivitytends to be poor even when it falls within this range. There are notmany moisture sensitive resistors that can measure a resistance at ahigh precision in a low humidity range.

Moisture sensitive resistors using apatite are also known. JapanesePatent Publication No. 60-35802 discloses a moisture sensitive resistorusing FAp, and Japanese Patent Disclosure (Kokai) Nos. 59-60348 and59-60350 disclose moisture sensitive resistors using hydroxyapatite.However, the resistances of these resistors corresponding to thehumidity are also high. In an attempt to decrease the resistance, someof the cations of apatites has been substituted by other cations (e.g.,Li, Na, K, Ag, and Cu). However, even after substitution, theresistances of these resistors are not sufficiently low. In sinteredhydroxyapatite, a hyteresis occurs in the resistance when measurement isperformed from high to low humidity side and vice versa, resulting inlow reliability.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a methodof producing FAp having a uniform composition without substantiallyaccompanying by-products.

In order to achieve this object, according to the present invention, amixture containing calcium monohydrogen phosphate, calcium carbonate,and calcium fluoride at a predetermined ratio is subjected to amechanochemical action, and is thereafter reacted to each other inwater.

It is another object of the present invention to provide a moisturesensitive resistor wherein a resistance corresponding to a humidity issufficiently low and a hysteresis does not substantially occur. In orderto achieve this object, in the present invention, FAp obtained by theabove producing method is sintered and then used as the moisturesensitive resistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a moisture sensitive element having amoisture sensitive resistor according to the present invention;

FIG. 2 is a graph showing an X-ray diffraction pattern of FAp obtainedby the method of the present invention;

FIG. 3 is a graph showing the moisture sensitive characteristic of themoisture sensitive resistor fabricated using FAp exhibiting the X-raydiffraction pattern shown in FIG. 2 together with those of ComparativeExamples;

FIG. 4 is a graph showing the hysteresis of the resistance of themoisture sensitive element using the moisture sensitive resistorexhibiting the moisture sensitive characteristic shown in FIG. 3;

FIG. 5 is a graph showing the moisture sensitive characteristic of themoisture sensitive resistors according to another embodiments of thepresent invention; and

FIG. 6 is a graph showing the moisture sensitive characteristic of themoisture sensitive resistors according to still another embodiments ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the FAp producing method of the present invention, calciummonohydrogen phosphate (CaHPO₄), calcium carbonate (CaCO₃), and calciumfluoride (CaF₂) are used as the raw materials. These materials are mixedat a predetermined ratio, or a substantially stoichiometric ratio (i.e.,CaHPO₄ :CaCO₃ :CaF₂ =6:3:1), and are subjected to a mechanochemicalaction. Then, these materials are caused to react with one another inwater (solid-water system reaction).

A mechanochemical action is what a solid material performs to have itsphysicochemical property changed by using part of the mechanical energyit has received (Kogyo Kagaku Zasshi Vol. 71, No. 9 (1968), p. 1301).The mechanical energy is applied to the solid body by means of grinding,friction, sliding, cutting, centrifuging, impact, and the like.

Such a mechanochemical action can be easily applied to the materials,for example, when the materials are mixed and ground by a ball mill. Atreatment using the ball mill can be performed at room temperature afterwater has been added to the mixture of raw materials. The grinding timeis normally 5 to 48 hours.

Subsequently, the materials are subject to a solid-water systemreaction. A solid-water system reaction is a reaction which utilizes thedifference in water solubility of a raw material system and a productsystem. Since the solubility of the materials is higher than that ofproduced FAp, once FAp is produced, it does not dissolve again butprecipitates. Therefore, a reaction equilibrium is always shifted to theproduct side and the FAp producing reaction progresses. Note that thetemperature at which this reaction occurs is 100° C. or less and ispreferably 80° to 100° C. With the conventional dry method, sincefluorine evaporates during the reaction, the composition becomes moreununiform. Generally, fluorine of fluorapatite begins to evaporate atabout 400° C., and the rate of evaporation increases as the temperatureincreases. In the present invention, however, since the reaction can beperformed at a low temperature, as described above, fluorine is notcaused to evaporate during the reaction and the composition does notbecome ununiform. The reaction time of this solid-water system reactionis normally 8 to 12 hours. This solid-water system reaction isquantitative and does not substantially produce by-products.

When the reaction is complete, FAp is recovered by means of filtrationand the like, and it is then dried. The thus-obtained FAp is a finepowder which has a particle size of 0.1 to 0.4 μm as measured by ascanning electron microscope, and has a good crystallinity. FAp producedwith this method has a good sintering property, and can, surprisingly,be sintered at a temperature as low as 700° to 800° C. so as to besuitable for a moisture sensitive resistor. This temperature which hasbeen range is lower than the temperature required for sintering FApwhich has been produced by the dry method (about 300° C.).

The moisture sensitive resistor of the present invention can be obtainedby sintering the FAp powder which has been produced by the above method.When electrodes are provided to this moisture sensitive resistor, amoisture sensitive element having an excellent moisture sensitivecharacteristic can be fabricated.

FIG. 1 is a sectional view of moisture sensitive element 10 havingmoisture sensitive resistor 11 of the present invention. Element 10 hasresistor 11 made of sintered FAp. Electrodes 12 and 13 are formed on thetwo surfaces of resistor 11 and connected to resistance measuringelement 14. Measuring element 14 measures the resistance of sensitiveelement 10. It is noted that the resistance of sensitive element 10corresponds to the relative humidity of the atmosphere, and thus thehumidity of the atmosphere can be determined.

It is generally known that when FAp is sintered, fluorine evaporates atabove 400° C., and FAp is caused to react with the atmospheric watermoisture at the vacancies which have been occupied by the evaporatedfluorine and is converted into hydroxyapatite. The higher the sinteringtemperature, the higher the degree of conversion. This conversion isundesirable since the logarithm of the resistance of hydroxyapatite ishigher than that of FAp by about 1, as is apparent from the Examplesdescribed below. Therefore, it is not advantageous to introduce ahydroxyl group which causes the conversion into a moisture sensitiveresistor, particularly when measurement in a low humidity range is to beperformed. For these reasons, FAp which can be sintered at a lowestpossible temperature is desired in manufacture a moisture sensitiveresistor. However, FAp manufactured in accordance with the conventionalmethod does not satisfy such a demand. In contrast to this, FApmanufactured in accordance with the method of the present invention hasa good sintering property, as described above, and can be sintered at atemperature which is lower than that of FAp produced by conventionalmethods.

FAp constituting the moisture sensitive resistor according to thepresent invention can be represented by a formula Ca₁₀ (PO₄)₆ F₂. It ispossible to substitute the calcium ions of the FAp within a range of 20atom % or less by at least one type of cations having an ion radius of0.95 Å (inclusive) to 1.35 Å (inclusive). These substituted FAp's canalso be used as the material of the resister. Some of the substitutedFAp's exhibit a lower resistance. Examples of such substituting cationsinclude a monovalent cation such as sodium, potassium, copper, andsilver; a divalent cation such as strontium, barium, titanium, chromium,manganese, cadmium, and lead; a trivalent cation such as indium,thallium, bismuth, and rare earth elements including, for example,scandium, yttrium, lanthanum and europium; and a tetravalent cation suchas zirconium and hafnium. Fluorapatite substituted by any of the abovecations can be obtained when calcium carbonate and calcium fluoride ofthe starting materials are partially substituted by a source of theselected type of cations (e.g., a carbonate and oxide) and subjected tothe same treatment as described above. When a carbonate is used as thecation source, the composition of the material is as follows:

    ______________________________________                                        Cation for   Composition of Material                                          substitution (molar ratio)                                                    ______________________________________                                        M+           CaHPO.sub.4 :CaCO.sub.3 :CaF.sub.2 :M.sub.2 CO.sub.3                          = 6:3 - X:1 - X:X (0 < X ≦ 1)                             M.sup.2+     CaHPO.sub.4 :CaCO.sub.3 :CaF.sub.2 :MCO.sub.3                                 = 6:3 - X:l:X (0 < X ≦ 2)                                 M.sup.3+     CaHPO.sub.4 :CaCO.sub.3 :CaE.sub.2 :M.sub.2 (CO.sub.3).sub.3                  2                                                                             = 6:3 - X:1 - X:X (0 < X ≦ 1)                             M.sup.4+     CaHPO.sub.4 :CaCO.sub.3 :CaF.sub.2 : M(CO.sub.3).sub.2                        = 6:3:1 - X:X (0 < X ≦ 0.5)                               ______________________________________                                    

The present invention will be described by way of its Examples.

EXAMPLE 1

Calcium monohydrogen phosphate, calcium carbonate, and calcium fluoridewere weighed at a molar ratio of 6:3:1 such that the synthesized amountof FAp was 50 g. The materials were mixed and ground using a ball milltogether with 200 ml of water for 24 hours at 80 rpm. The ground mixturewas caused to react in water at 100° C. for 10 hours to obtain FAp. Theobtained FAp was a fine powder having an average particle size of 0.2 μmas measured by a scanning electron microscope. FIG. 2 shows the X-raydiffraction pattern of this FAp.

EXAMPLE 2

Polyvinyl alcohol was added to the FAp powder obtained in Example 1 andsubjected to granulation. The resultant granules were press molded tohave a consolidated density of 1.5 g/cm³, thus preparing a tablet havinga diameter of 18.15 mm and a thickness of 1.0 mm. This tablet wassintered at 780° C. for 2 hours, thus obtaining a desired sintered body.Silver paste was coated on the two surfaces of the sintered body byscreen process printing and the sintered body was baked at 530° C.,thereby forming electrodes. A moisture sensitive element was thusobtained.

The moisture sensitive characteristic (relationship between thelogarithm of resistance [R] and relative humidity [RH]) of this moisturesensitive element is shown in FIG. 3. The measurement was conducted at25° C., 1 kHz, and 1 V.

The resistance (R [Ω]) of the moisture sensitive element made ofsintered FAp obtained in this example was measured both when moisteningwas performed to increase a relative humidity (RH) from 30 to 90% andwhen dehumidification was performed to decrease a relative humidity from90 to 30%. The measurement was conducted at 25° C., 1 kHz, and 1 V.Table 1 (Process of Moistening) and Table 2 (Process ofdehumidification) below show the measurement results. FIG. 4 shows ahysteresis curve obtained from the measurement results. Note that theresistance is indicated as a logarithm in FIG. 4

                                      TABLE 1                                     __________________________________________________________________________    (Process of Moistening)                                                       RH 30%     45%   60%     75%   90%                                            __________________________________________________________________________    R  5.80 × 10.sup.4                                                                 6.58 × 10.sup.3                                                               1.75 × 10.sup.3                                                                 6.50 × 10.sup.2                                                               2.62 × 10.sup.2                          __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    (Process of Dehumidification)                                                 RH 90%     75%   60%     45%   30%                                            __________________________________________________________________________    R  2.62 × 10.sup.2                                                                 6.69 × 10.sup.2                                                               1.69 × 10.sup.3                                                                 6.61 × 10.sup.3                                                               5.34 × 10.sup.4                          __________________________________________________________________________

COMPARATIVE EXAMPLE 1 Fluorapatite Synthesized by a Dry Method

Herein, calcium monohydrogen phosphate, calcium carbonate, and ammoniumfluoride were caused to react with one another by mixing them at a molarratio of 6:4:1, placing them in a crucible which is open to the air, andheating at 950° C. for 4 hours to prepare FAp. A moisture sensitiveelement was then obtained by using the obtained FAp and following thesame procedures as in Example 2, except that the consolidated densityand sintering temperature were respectively set at 1.9 g/cm³ and 1,200°C. The moisture sensitive characteristic of this element is shown inFIG. 3.

COMPARATIVE EXAMPLE 2 Hydroxyapatite

In this example the same operation was followed as in Example 1, exceptthat calcium monohydrogen phosphate and calcium carbonate were weighedand mixed at a molar ratio of 6:4, and hydroxyapatite was obtained. Amoisture sensitive element was obtained using the obtainedhydroxyapatite and following the same operation as in Example 2 exceptthat sintering was performed at 900° C. for 6 hours. The moisturesensitive characteristic of this element is shown in FIG. 3.

As is apparent from FIG. 3, the moisture sensitive resistor of thepresent invention has a lower resistance than moisture sensitiveresistors which are made of sintered fluorapatite synthesized by the drymethod or of sintered hydroxyapatite.

As is apparent from FIG. 4, hysteresis does not substantially occur inthe resistance of the moisture sensitive resistor of the presentinvention.

EXAMPLE 3

Herein, calcium monohydrogen phosphate, calcium carbonate, calciumfluoride, and potassium carbonate were weighed at a molar ratio ofCaHPO₄ :CaCO₃ :CaF₂ :K₂ CO₃ =6:2.5:0.5:0.5. A moisture sensitive elementwas obtained using these powdery materials in accordance with the samemethod as used in Example 2. FIG. 5 shows the moisture sensitivecharacteristic of this element. The measurement conditions were 25° C.,1 kHz, and 1 V. This result shows that when FAp calcium ions arepartially substituted by a predetermined type of cations, the resistanceof the moisture sensitive element corresponding to the relative humidityis further decreased.

EXAMPLE 4

In this example the same operation as carried out in Example 2 wasperformed except that calcium monohydrogen phosphate, calcium carbonate,calcium fluoride, and sodium carbonate were used as the raw materials ata molar ratio of CaHPO₄ :CaCO₃ :CaF₂ :Na₂ CO₃ =6:2.5:0.5:0.5, and thatthe sintering temperature was set at 900° C. The moisture sensitivecharacteristic of a moisture sensitive element made of a sintered bodyof the obtained Na-substituted FAp (FAp(Na)) is shown in FIG. 6.

EXAMPLE 5

In this example the same operation as used in Example 2 was performedexcept that calcium monohydrogen phosphate, calcium carbonate, calciumfluoride, and strontium carbonate were used as the raw materials at amolar ratio of CaHPO₄ :CaCO₃ :CaF₂ :SrCO₃ =6:2:1:1, and that thesintering temperature was set at 840° C. The moisture sensitivecharacteristic of a moisture sensitive element made of a sintered bodyof the obtained Sr-substituted FAp (FAp(Sr)) is shown in FIG. 6.

EXAMPLE 6

In this example the same operation as followed in Example 2 wasperformed except that calcium monohydrogen phosphate, calcium carbonate,calcium fluoride, and barium carbonate were used as the raw materials ata molar ratio of CaHPO₄ :CaCO₃ :CaF₂ :BaCO₃ =6:2:1:1. The moisturesensitive characteristic of a moisture sensitive element made of asintered body of the obtained Ba-substituted FAp (FAp(Ba)) is shown inFIG. 6.

EXAMPLE 7

In this example the same operation as carried out in Example 2 wasperformed except that calcium monohydrogen phosphate, calcium carbonate,calcium fluoride, and lanthanum carbonate were used as the raw materialsat a molar ratio of CaHPO₄ :CaCO₃ :CaF₂ :La₂ (CO₃)₃ =6:2.5:0.5:0.5, andthat the sintering temperature was set at 1,000° C. The moisturesensitive characteristic of a moisture sensitive element made of asintered body of the obtained La-substituted FAp (FAp(La)) is shown inFIG. 6.

EXAMPLE 8

In this example the same operation as followed in Example 2 wasperformed except that calcium monohydrogen phosphate, calcium carbonate,calcium fluoride, strontium carbonate, and barium carbonate were used asthe raw materials at a molar ratio of CaHPO₄ :CaCO₃ :CaF₂ :SrCO₃ :BaCO₃=6:1:1:1:1, and that the sintering temperature was set at 900° C. Themoisture sensitive characteristic of a moisture sensitive element madeof a sintered body of the obtained Sr, Ba-substituted FAp (FAp(Sr, Ba))is shown in FIG. 6.

As described above, according to the present invention, an FAp having auniform composition can be produced without substantially produceby-products.

The moisture sensitive resistor according to the present invention has asufficiently low resistance corresponding to a humidity andsubstantially no hysteresis in the resistance.

What is claimed is:
 1. A method of producing fluorapatite comprising thesteps of:(a) providing a mixture containing calcium monohydrogenphosphate, calcium carbonate, and calcium fluoride substantially in aratio of CaHPO₄ :CaCO₃ :CaF₂ of 6:3:1; (b) subjecting the mixture to amechanochemical action selected from the group consisting of grinding,friction, sliding, cutting, centrifuging, and impact; (c) causing themixture to react in water at a reaction temperature of at most 100° C.thereby depositing fluorapatite; and (d) collecting the depositedfluorapatite from the mixture.
 2. A method according to claim 1, whereinraw materials are mixed and ground using a ball mill, thereby subjectingthe raw materials to the mechanochemical action.
 3. A method accordingto claim 1, wherein the reaction in water is performed at a temperatureof 80° to 100° C.